Hybrid electric vehicles use multiple prime movers for optimum fuel economy. An internal combustion engine provides input torque to a transmission at higher vehicle speeds, i.e., when the engine is relatively efficient. At vehicle launch and while traveling below a threshold vehicle speed, one or more high-voltage electric traction motors may provide the required input torque. Such traction motors draw power from a rechargeable battery pack. Motor torque from the electric traction motor(s) is also used as needed to assist the engine in propelling the vehicle, e.g., during transient periods of increased torque request. Hybrid electric vehicles typically include other fuel saving features such as automatic engine shutoff at idle and regenerative braking